Cantilever tools are characterized by the cutting instrument being spaced from the longitudinal axis so that forces generated during cutting tend to cantilever the boring bar in a direction perpendicular to axis. During either rotation of the tool or workpiece, the bending moment along the axis tends to displace the cutting tool in the radial direction, hence, making it difficult to perform accurate cutting.
In an article entitled "An Extra-Long Cantilever Boring Bar With Enhanced Chatter Resistance" by Rivin and Wu, Department of Mechanical Engineering, Wayne State University, Detroit, Michigan, prior art techniques for enhancing the chatter-resistance of cantilever tools are discussed. As set forth in line 15 of the introduction the article, "Both resonance frequencies and chatter resistance are correlated with the length (L) to diameter (D) ratio of the cantilever holders. All practitioners know that it is difficult to achieve stable cutting at overhang ratios exceeding L/D=4-5 [4-5:1]. While this ratio is not high, one has to remember that to machine (or inspect) a hole, the diameter D of the tool should be smaller than the hole diameter to accommodate cutting (or measuring) head, but the length L of the tool should be longer than the hole depth to eliminate danger of interference between the spindle holding the tool and the part being machined. In automated tool handling systems (machining centers, flexible manufacturing cells, etc.) the useful length of tools is further reduced by the need for special design features such as gripping surfaces for tool changers, surfaces for identification codes, etc.
As set forth, more stable chatter-resistant tools make possible deeper cuts, and thus fewer cuts, resulting in higher productivity. Chatter-resistant tools also combine better surface finish with more intensive machining regimes. They demonstrate slower wear of cutters due to reduced high-frequency vibration amplitudes, which may result in better cylindricity of machined surfaces.
The article describes the boring bar fabricated for the experiments carried out. The boring bar includes an aluminum end segment having an inside cavity for receiving dynamic vibration absorbers. A connecting lug threaded into the aluminum section joins a cemented carbide stiffener. The tool head is mounted on the aluminum segment and includes a cutting insert displaced from the axis of rotation.
In an attempt to place a highly elastic material near the cutting insert, the boring bar of Rivin utilizes an aluminum section. For long life, it has been found that aluminum does not have the requisite structural integrity and can be easily damaged during multiple cutting operations. Such damage to the aluminum section can result in a wearing away of the aluminum thus creating an imbalanced tool. Aluminum does not have the requisite properties to hold the head on the bar with threading or to hold the insert in the pocket. Thus, frequent replacement of the aluminum head is necessitated.
In another article entitled "A Chatter-Resistant Cantilever Boring Bar" by Rivin, the stability of various boring bars was examined. The results of tests show that the tungsten alloy and aluminum bar incorporating dynamic vibration absorbers performed better than a solid steel bar, a solid steel bar with dampening vibration absorbers, a solid bar of machinable tungsten alloy, or composite bar without the dampening vibration absorbers. The article concludes that the use of composite cantilevered structure having a root portion of high-modulus materials with an overhand portion of low-density materials improves dynamic characteristics, especially if dynamic vibration absorbers are employed.
U.S. Pat. No. 3,820,422 to Rivin relates to a composite cantilevered tool mandrel having two portions with the portion adjacent the tool having a modulus of elasticity of at least 1.5 times higher than that of the other portion. Further, the head portion has a material density of at least 1.5 times lower than the rear portion. As set forth, the combination of materials for the portions include steel plus aluminum or titanium alloy, tool hardened alloy plus steel, or molybdenum plus aluminum. As set forth, the natural elastic properties of the materials are utilized with the bar portion near the cutting tool being highly elastic and less dense than the driving portion of the bar which is rigid. As set forth in the patent, the connection between the two sections is made by a thread type or interference connection. The vibration shock absorber utilized in the head portion is a solid cylindrical slug.
Thus, Riven concentrates on a boring bar where a low density and highly elastic head portion is confined with a high density and extremely rigid portion. U.S. Pat. No. 3,923,414 to Hopkins relates to a vibration dampening support for a boring bar utilizing multiple parallel bores and a plurality of inertia mass members positioned in the respective bores with a slight circumferential clearance. The mass members are free to move transversely to the shank axis within the circumferential clearance provided.
U.S. Pat. No. 3,938,626 to Hopkins relates to a vibration dampening construction for a cutter head having a rigidly anchored cantilever internal bar extending rearwardly within a cylindrical cavity. A optional number of heavy reactors are assembled on the cantilevered bar to give a reactive oscillator dampening effect.
The present invention concerns the production of an optimized boring bar constructed of materials which contribute to prolong tool life and are structurally combined in such a manner so as to give enhanced performance.